1. Field of the Invention
The invention relates to a copolymer, and in particular to an aliphatic copolyester and preparation method thereof.
2. Description of the Related Art
Lactic acid monomer is isolated from fermented corn. Biodegradable polylactic acid (PLA) polymerized by lactic acid is a prospective and important environmental protection material, capable of application to various fields, for example, biomedicine, agriculture, fishery, plastic and textile industry. The water absorption rate of polylactic acid fiber is 0.6% lower than nylon fiber (4.5%), similar to polyesters such as polyethylene terephthalate (PET) (0.4%). The water absorption rate or hydrophilicity affects decomposition rate thereof. Higher hydrophilicity results in faster decomposition rate. Additionally, other products, for example, a drug delivery carrier, tissue engineering material or agricultural packaging, require using hydrophilic polylactic acid. For textiles, higher hydrophilicity brings better comfort. Thus, improvement of hydrophilicity of polylactic acid is desirable to meet future requirements of various product fields.
Hydrophilic polyethylene glycol (PEG) has been approved by the FDA for usage in the human body. Hydrophilicity of polylactic acid can be improved by conduction of PEG thereinto. Additionally, due to residual solvent and cost considerations, the non-solvent melt fabrication method is the current raw material polymerization technique. JP. Pat. 09-031179 discloses mixing polylactic acid and PEG. However, mixture leakage and phase separation occur because of deteriorated compatibility between polylactic acid and PEG. JP. Pat. 2004-231773 and 2001-323052 disclosing copolymerization of hydrophilic PEG and lactide to prepare a hydrophilic polylactic acid copolymer can overcome the aforementioned drawback. First, lactic acid and PEG are polycondensed to prepare a low-molecular-weight hydrophilic lactic acid copolymer containing PEG located at a main chain thereof, with a molecular weight of 600-10,000 g/mole. The hydrophilic oligomer and lactide are then copolymerized by ring-open polymerization to prepare a diblock or triblock lactic acid copolymer containing PEG located at the main chain thereof. However, the product contains high-molecular-weight block polylactic acid, bearing inherent drawbacks of polylactic acid.
Adding catalyst to polycondensation of lactic acid and PEG to prepare an alternative hydrophilic lactic acid copolymer containing PEG located at main chain thereof taught by Z. Y. Wang can overcome the aforementioned drawback. However, the molecular weight of the product is merely 41,000 g/mole. To achieve practical requirement, the molecular weight thereof must be increased. Additionally, U.S. Pat. Nos. 5,359,026 and 6,730,772 disclose ring-open polymerization of hydrophobic peroxide monomer and lactide to prepare a high-molecular-weight polylactic acid copolymer containing the hydrophobic monomer located at a side chain thereof, with a molecular weight exceeding 150,000 g/mole. However, it is difficult to perform melt copolymerization by directly mixing hydrophobic lactide and a hydrophilic monomer with a molecular weight exceeding 200 g/mole. U.S. Pat. No. 6,221,977 discloses ring-open polymerization of lactide and a hydrophilic monomer with a molecular weight exceeding 200 g/mole to prepare a high-molecular-weight polylactic acid copolymer containing the hydrophilic monomer located at a side chain thereof, with a molecular weight exceeding 150,000 g/mole. However, the spinning rate of the product is low (less than 800 m/min) in spinning tests. Additionally, after the polylactic acid copolymer is purified by the solvent precipitation method, the hydrophilic PEG content of the polylactic acid with a molecular weight exceeding 100,000 g/mole is less than 0.7 wt %. Most of hydrophilic PEG locates at the oligomer with a molecular weight less than 50,000 g/mole.